1. Field of the Invention
The present invention relates not only to a functional optical element suitably adapted to be used in various devices for optical recording, optical coupling, optical communications, optical computation, optical display and so on but also a functional optical device comprises the optical elements of the type described above and more particularly an optical element and an optical device each having a diffraction grating which is not dependent upon the plane of polarization of a light beam used.
2. Related Background Art
So far various diffraction gratings have been used as functional optical elements in, for instance, spectroscopes, wave branching devices, wave synthesizing devices or reflectors. In addition, they are recently used in semiconductor lasers and optical integrated circuits and become very important optical elements.
Of the various diffraction gratings, a phase type diffraction grating has its surface roughened or varies the index of refraction in the medium in order to vary the phase. In general, the medium used in the diffraction grating of the type described above comprises an optically isotropic material and accomplishes its inherent function independently of the polarization of the light beam used. However, recently there has been an increasing tendency that the diffraction grating is made of an optically anisotropic material such as a crystal having an optic axis extended in a predetermined direction. In this case, the characteristic of the diffraction grating changes depending upon the direction of polarization of the light beam used. As a result, except that the case in which a light source is a laser and a plane polarized light beam is used, it is needed to polarize the light beam through a diffraction plate in a predetermined direction so that the optical efficiency is remarkably decreased in this step.
When the functional optical device utilizing the above-described optically anisotropic substance such as a liquid crystal, PLZT, BSO or other electro-optical crystals is used, the problems similar to those described above arise. In the device utilizing an electro-optical crystal, comb-shaped electrodes are disposed on the surface of a sliced electro-optical crystal in such a way that they cross each other, and a polarizer and a photosensor are disposed in front of and behind the electro-optical crystal. When an electric field is applied to the comb-shaped electrodes the index of double refraction of the crystal is varied, thereby controlling the light beam passing through the device comprising the polarizer, the electro-optical crystal and the photosensor. Such device as described above has a relatively high response characteristic and a high degree of contrast ratio in the case of a monochrome light beam, but has some defects that the driving voltage is considerably high ranging from 100 V to a few kV and it is difficult to attain a device having a large surface area.
In a device utilizing a liquid crystal, a liquid crystal is sandwiched between transparent electrodes disposed perpendicular to each other, and is oriented helically so that in the static state, the light beam passes through the mutually perpenducular electrodes, but when an electric field is applied, the liquid crystal is oriented in the direction of the applied electric field so that the light beam is interrupted by the polarizer disposed on the output side, whereby the transmission of the light beam through the device is cut off. The device utilizing such liquid crystal of the type described above can be driven at a relatively low driving voltage and the costs of the component parts are inexpensive. However, such device has a slow switching response speed and is instable in operation in response to the variation of temperature. In addition, its optical efficiency and contrast ratio are not satisfactory in practice.
As described above, the prior art functional optical devices have their own merits and demerits and a common problem similar to that encountered in the prior art functional optical elements. That is, these devices use as an incident light beam, a light beam having a specific polarization characteristic and in general a plane polarized light beam. As a result, in the case of the incident light which is polarized in random directions, polarizers must be used so that the optical efficiency is considerably reduced when the incident light beam passes through the polarizers.